There are a number of known active vehicle suspension systems which actively control the vertical relative movement between each wheel and the vehicle body with a linear hydraulic actuator according to the condition of the road surface and the movement of the vehicle body. An example of an active vehicle suspension system is disclosed in Japanese patent laid open publication No. 62-1611.
A typical linear hydraulic actuator employed in such an active vehicle suspension system comprises a piston slidably received in a cylinder, defining two hydraulic chambers, and a desired relative movement between the piston and the cylinder is produced by the difference in the hydraulic pressures present in these two chambers. Normally, the piston is provided with a piston rod which extends axially from one end of the cylinder so that the piston presents two pressure receiving surfaces having different areas to the two hydraulic chambers.
Typically, a bottom end of the cylinder is coupled to the wheel and an outer end of the piston rod is coupled to the vehicle body. Further, the output from a hydraulic pump is directly supplied to one of the hydraulic chambers (upper chamber) exposed to the surface receiving surface of the piston having the smaller area, and a feedback control is carried out on the pressure of the other hydraulic chamber (lower chamber) via a pressure control valve according to the displacement of the hydraulic actuator and the change rate of the displacement so that the wheel may closely follow the road surface and the height of the vehicle body may be maintained at a fixed level.
However, since the pressure control valve normally consisting of a solenoid valve involves a certain response delay, the vehicle suspension system may not be able to adapt itself to a high frequency component of the input from the road surface, and a surging may be developed in the pressure applied to the lower chamber in extreme cases. To eliminate this problem, in most active vehicle suspension systems, a gas-filled accumulator is connected to the line between the pressure control valve and the lower chamber so that the high frequency component of the pressure change the lower chamber may be passively absorbed by the accumulator. Additionally or alternatively, a relief valve is connected to this line so that excessive pressure in the hydraulic actuator may be safely vented.
In such a hydraulic system, the presence of an accumulator in the output line of the pressure control valve tends to impair the response speed of the system, and increasing the response speed of the system requires a pump having a relatively large capacity. Further, a relief valve is not capable of venting excess pressure unless there is a substantial difference in pressure between the upper and lower chambers.